Such a device is known from patent publication US 2008/0265753-A1. This document describes typical OLEDs having a substrate, a first electrode layer being deposited directly on the substrate, an electroluminescence layer (EL) and a second electrode layer being deposited on the EL. Said EL is composed of different sublayers, including an emission layer and a conductive layer. Upon applying a voltage in the range of 2-10 V between both electrode layers, the EL emits light. Depending on the type of the electroluminescence materials used in the EL, different light spectrums (colors) can be generated with OLEDs. At least one of the two electrode layers is transparent for light generated by the EL. The described layer package is contained in a cavity formed by a bowl-like cover layer. Said cover is secured at its rim to the glass substrate by means of a sealing material.
The known OLED also comprises moisture-absorbing means positioned at different locations in the cavity, like inside the sealing material, on the second electrode layer and on the inner surface of the cover layer. These moisture-absorbing means function as a getter for chemically or physically removing moisture from the inside of the OLED. Said moisture-absorbing means may comprise water-absorbing materials like metaloxide particles (f.e. CaO) being bound in a matrix of an organic polymer. In OLEDs, the anode/EL/cathode package is known to be extremely sensitive to moisture. Therefore, the presence of moisture-absorbing means has been shown to be essential in enhancing the life-span of OLEDs. If possible, the level of moisture should remain below 1000 ppm.
The described OLED further comprises a separating foil, which is positioned between the cover layer and the moisture-absorbing means which is deposited on the second electrode layer. Said separating foil acts as a compliant protection layer which should prevent direct contact between the cover and the EL. Such contact can cause mechanical damage to one the electrodes or shorting. In the described OLED, mechanical stresses due to flexing are absorbed by the separating foil of resilient material.
The here-described OLED is of the so-called ‘cavity-lid’ type, in which the EL is contained in a bowl-like cover. In principle, light generated by the EL exits the OLEDs of the cavity-lid type only via the substrate. Therefore, both the substrate and the first electrode layer deposited on it are transparent to the light generated by the EL. Often the substrate is made of glass, the first electrode layer of ITO (Indium Tin Oxide, or InSnO) and the second electrode of a thin layer of Al. The cover can be made of glass or metal. Another type of OLEDs is commonly referred to as the ‘flat-lid’ type. In this type, the mentioned layer package of the EL is sandwiched between the first and second electrode layers. Latter layers are covered on both main surfaces by flat substrates. In this OLED type, both substrates and both electrode layers can be more or less transparent for the light generated by the EL. In OLEDs of the flat-lid type, the substrate and the cover are often made of glass whereas the first electrode is made of ITO and the second electrode can be formed as a thin layer of Al.
It has been experienced that the known OLED shows certain drawbacks. It has appeared that, also in the absence of any mechanical pressures on their substrates, the known OLEDs show too many failures. This is especially observed during so-called ‘drop tests’, in which the OLEDs are dropped from a limited height (f.e. 1 meter) on the floor. Under these circumstances, the EL stops emitting light under appropriate voltages. As a consequence, the current mean life-span of the known OLEDs is considered to be still insufficient.